When boring a hole into the earth it is customary to use a drilling rig including a vertically oriented derrick and a rotary power head. Lengths of drill rod, each having a pin end (male thread) and a box end (female thread) are connected together and rotated by the power head to advance a drill bit, connected to the lowermost drill rod, into the earth. As the drill bit advances downwardly, the power head is lowered within the drill rig until it reaches its lowermost point, at which time it must be disconnected from the last section of drill rod, raised, and connected to a new section of drill rod which in turn is connected to the last section of drill rod in the drill string.
The previous method of threading together the threaded connections on a drill string involved the rotation of the power head at the same time as the male and female threads were brought together. The main problem with this method was the difficulty in coordinating the pitch of the mating threads in the horizontal plane with the vertical positioning of the thread pitch, as the two thread pitches were essentially solidly positioned apart except for the up-down feed. It was therefore difficult to start the threads in the correct pitch sequence and as a result the threads were often cross-threaded.